1. Field of the Invention
The present invention relates to improvements in an ignition timing control system for an internal combustion engine, and more particularly to an ignition timing control system for the purpose of stabilizing engine revolution, incorporated with an air-fuel ratio feedback control system.
2. Description of the Prior Art
Most automotive internal combustion engines have been equipped with an air-fuel ratio feedback control system to feedback-control an air-fuel ratio to a stoichiometric value in order to effectively activate a three-way catalytic converter. The air-fuel ratio control system includes an air-fuel ratio sensor (an oxygen sensor in practice) which is disposed in an exhaust gas passageway to detect the concentration of oxygen in exhaust gas and produce a rich-lean signal representative of the air-fuel ratio being in a rich or lean state. In response to this signal, an air-fuel ratio feedback control coefficient .alpha. is set under a known proportional plus integral control. A basic fuel injection amount Tp determined corresponding to the amount of intake air to be inducted into the engine is corrected with the air-fuel ratio feedback correction coefficient .alpha. thereby obtaining a fuel injection amount Ti (=Tp.times..alpha.) to feedback-control the air-fuel ratio to the stoichiometric value.
In such an air-fuel ratio feedback control system, the air-fuel ratio feedback correction coefficient .alpha. changes periodically in response to the periodical rich-lean signal from the oxygen sensor. Accordingly, assuming that the basic fuel injection amount Tp is constant, the fuel injection amount Ti takes its maximum value at the maximum value of the air-fuel ratio feedback correction coefficient .alpha., whereas it takes its minimum value at the minimum value of the air-fuel ratio feedback correction coefficient .alpha.. Under such changes in the fuel injection amount Ti, fluctuation in engine revolution will occur, which is particularly enormous at engine idling.
In view of the above, it has been proposed to control an ignition (spark) timing control system in relation to the air-fuel ratio control system, for example, disclosed in Japanese Patent Provisional Publication No. 61-98970. In this proposition, as shown in FIG. 9, a deviation (.alpha.-.alpha..sub.AVE) between the feedback correction coefficient .alpha. and a moving average .alpha..sub.AVE of the air-fuel ratio feedback correction coefficient .alpha. is determined, and then an ignition timing is correct-controlled in accordance with the deviation in such a manner as to be in a retarded side in an amount corresponding to the magnitude of the deviation when the deviation is positive while in an advanced side in an amount corresponding to the magnitude of the deviation when the deviation is negative, thereby intending stabilization of engine revolution at engine idling.
However, drawbacks have been encountered in the above-discussed conventional control manner of the ignition timing control system in corporation with the air-fuel ratio feedback control system, as discussed hereinafter. First, a consideration will be made on actual air-fuel ratio of each engine cylinder of an internal combustion engine upon change in the fuel injection amount. As shown in FIG. 10A, when the amount (an fuel injection amount) of fuel to be injected to a certain cylinder changes by an amount (.DELTA.fuel=Tp.times..DELTA..alpha. where .DELTA..alpha. is a step change amount of the air-fuel ratio feedback correction coefficient .alpha.) as shown in FIG. 10B, the amount of fuel to be introduced into the cylinder changes along a broken line in FIG. 10B with the number of times of air intake in the cylinder because a part of the injected fuel forms fuel flow on the wall surface of an air intake passageway. As clearly seen from FIG. 10B, the response characteristics of the fuel to be supplied to the cylinder is regarded to have a time lag characteristic with which the amount of fuel to be sucked to the cylinder gradually increases with the number of times of air intake.
Accordingly, when the air-fuel ratio feedback correction coefficient .alpha. changes as shown in FIG. 11A corresponding to an air-fuel ratio (A/F) change detected by the oxygen sensor in the exhaust gas passageway, the actual air-fuel ratio (cylinder A/F) in each cylinder changes as shown in FIG. 11D which indicates an example of the first cylinder (#1) from an end of the engine. It will be seen that the air-fuel ratio in each cylinder does not take a change manner corresponding to that of the air-fuel ratio feedback correction coefficient .alpha., and therefore the change in the cylinder air-fuel ratio has a time lag characteristic (for the reason of FIG. 10B relative to the change in the air-fuel ratio feedback correction coefficient .alpha.). Consequently, when the air-fuel ratio feedback correction coefficient .alpha. changes, for example, from a decreasing side to an increasing side, the air-fuel ratio in each cylinder cannot immediately come to the rich side, shortly keeping it in the lean side.
In view of the above, with the above-discussed conventional control manner for the ignition timing control system, correction of the ignition timing is made in accordance with the air-fuel ratio feedback correction coefficient. As a result, the following shortcomings may occur: The ignition timing is unavoidably retarded though the actual air-fuel ratio in each cylinder is in the lean side, whereas it is unavoidably advanced though the actual air-fuel ratio is in the rich side, thereby rather increasing an engine revolution fluctuation.
Another conventional control manner for an ignition timing control system has been proposed, for example, in Japanese Patent Provisional Publication No. 60-56149, in which the ignition timing is retarded in response to the judgment (due to oxygen sensor output) of the air-fuel ratio being in the rich side, and it is advanced in response to the judgment of the air-fuel ratio being in the lean side. Thus, this control manner does not take account of a time lag of exhaust gas flow from the cylinder to the oxygen sensor and the retarded response of the oxygen sensor and therefore, an actual air-fuel ratio in each cylinder cannot be detected, generating a problem in that the ignition timing is unavoidably retarded even though the actual air-fuel ratio in each cylinder is in the lean side.